Packing materials for liquid chromatography (LC) are generally classified into two types: organic materials, e.g., polydivinylbenzene, and inorganic materials typified by silica. Many organic materials are chemically stable against strongly alkaline and strongly acidic mobile phases, allowing flexibility in the choice of mobile phase pH. However, organic chromatographic materials generally result in columns with low efficiency, leading to inadequate separation performance, particularly with low molecular-weight analytes. Furthermore, many organic chromatographic materials shrink and swell when the composition of the mobile phase is changed. In addition, most organic chromatographic materials do not have the mechanical strength of typical chromatographic silicas.
Due in large part to these limitations, silica is the material most widely used in High Performance Liquid Chromatography (HPLC). The most common applications employ silica that has been surface-derivatized with an organic functional group such as octadecyl (C18), octyl (C8), phenyl, amino, cyano, etc. As stationary phases for HPLC, these packing materials result in columns that have high efficiency and do not show evidence of shrinking or swelling.
Silica is characterized by the presence of silanol groups on its surface. During a typical derivatization process such as reaction with octadecyldimethylchlorosilane, at least 50% of the surface silanol groups remain unreacted. These residual silanol groups interact with basic and acidic analytes via ion exchange, hydrogen bonding and dipole/dipole mechanisms. The residual silanol groups create problems including increased retention, excessive peak tailing and irreversible adsorption of some analytes. Another drawback with silica-based columns is their limited hydrolytic stability. First, the incomplete derivatization of the silica leaves patches of bare silica surface which can be readily dissolved under alkaline conditions, generally pH>8.0, leading to the subsequent collapse of the chromatographic bed. Secondly, the bonded phase can be stripped off the surface under acidic conditions, generally pH<2.0, and eluted off the column by the mobile phase, causing loss of analyte retention, and an increase in the concentration of surface silanol groups.
To overcome the problems of residual silanol group activity and hydrolytic instability of silica-based stationary phases, many methods have been tried including use of ultrapure silica, carbonized silica, coating of the silica surface with polymeric materials, endcapping free silanol groups with a short-chain reagent such as trimethylsilane, and the addition of suppressors such as amines to the eluant. These approaches have not proven to be completely satisfactory in practice.
One approach is disclosed in U.S. Pat. No. 4,017,528. A process for preparing a “hybrid” silica is described wherein an alkyl functionality is coupled into both the skeleton structure and the surface of the silica. According to the '528 patent, the hybrid silica can be prepared by two methods. In the first method, a mixture of tetraethoxysilane (TEOS) and an organotriethoxysilane, e.g., alkyltriethoxysilane, is co-hydrolyzed in the presence of an acid catalyst to form a liquid material containing polyorganoethoxysiloxane (POS) oligomers, e.g., polyalkylethoxysiloxane oligomers. Then, the POS is suspended in an aqueous medium and gelled into porous particles in the presence of a base catalyst. In the second method, the material is prepared by a similar procedure except that the suspension droplet is a mixture of organotriethoxysilane, e.g., alkyltriethoxysilane, and polyethoxysiloxane (PES) oligomers; the latter is prepared by partial hydrolysis of TEOS.
There are several problems associated with the '528 hybrid material. First, these hybrid materials contain numerous micropores, i.e., pores having a diameter below 34 Å. It is known that such micropores inhibit solute mass transfer, resulting in poor peak shape and band broadening.
Second, the pore structure of the '528 hybrid material is formed because of the presence of ethanol (a side product of the gelation process) within the suspension oil droplets. The pore volume is controlled by the molecular weight of the POS or PES. The lower the molecular weight of the POS or PES, the more ethanol is generated during the gelation reaction, and subsequently a larger pore volume is produced. However, part of the ethanol generated during the gelation is able to diffuse into the aqueous phase by partition. If the amount of the ethanol generated within the suspension droplets is too great, the partition of the ethanol will cause the structure of the droplets to collapse, forming irregularly-shaped particles as opposed to spherical particles. Therefore, the strategy to control the pore volume of the hybrid material described in the '528 patent has certain limitations, particularly for preparing highly spherical hybrid materials with a pore volume greater than about 0.8 cm3/g. It is well known in the art that irregularly-shaped materials are generally more difficult to pack than spherical materials. It is also known that columns packed with irregularly-shaped materials generally exhibit poorer packed bed stability than spherical materials of the same size.
Thirdly, the '528 hybrid materials are characterized by an inhomogeneous particle morphology, which contributes to undesirable chromatographic properties, including poor mass transfer properties for solute molecules. This is a consequence of the gelation mechanism, where the base catalyst reacts rapidly near the surface of the POS droplet, forming a “skinned” layer having very small pores. Further gelation in the interior of the droplet is then limited by the diffusion of catalyst through this outer layer towards the droplet center. leading to particles having skeletal morphologies and hence pore geometries, e.g., “shell shaped”, which can vary as a function of location between the particle center and outer layer.